a. hear someone say ~ The only way i can think of hearing this would be in a sentence like: Did you hear someone say that class is over? I heard someone say that there is no class tomorrow. b. hear someone says ~ Cannot say this, only "hear someone say". You can however say, "Someone says", "If someone says we should go, then we should go!" my english grammar is not good enough to give you a very detailed scientific reason, but this is how we speak :) c. hear someone saying ~ It's really the same thing as "hear someone say". You can use this if you want to emphasize the fact that you were listening to them at the same time they were speaking. " I heard someone saying that there is no class tomorrow" (means you must have been there at that time, while they were speaking ). " but really, it's no different from "I heard someone say". 99% same. There may be a case where you'd prefer to say "saying' over "say" - but I can't think of what the difference is. really, you use 'saying' to put emphasis on the fact that you were listening to them speak, rather than just having heard them say something? hope that helps...

Ian's answer is perfect.

I think the first two is the same. it menas that someone says something usually the third one is maybe I just heard someone saying something so it happend just a momeng ago